Dual-Polarization DOW Radar Data Analysis of the Impact of Ground-Based Glaciogenic Seeding on Winter Orographic Clouds

The efficiency of silver iodide (AgI) seeding of cold-season orographic clouds and the related microphysical processes are still poorly understood. In order to investigate both natural snow growth processes and seeding-induced microphysical changes in orographic clouds, the AgI Seeding Cloud Impact Investigation (ASCII) project was conducted in southern Wyoming in 2012-2013. As part of ASCII, a Doppler-on-Wheels (DOW) X-band dual-polarization Doppler radar operated on Battle Pass in the Sierra Madre in January-March 2012. This poster analyses the impact of ground-based glaciogenic seeding on winter orographic clouds by means of DOW data, collected in nine Intensive Observation Periods (IOPs). The IOPs are classified to two categories: in six cases the precipitation was mostly stratiform, and in three cases relatively shallow convection dominated or was embedded in stratiform precipitation. In order to examine the seeding effect, three study areas are defined (an upwind control area, a short-fetch target area, and a more remote target area) for the mostly-stratiform cases. An extra lateral control area (over the mountain range, but sideways of the generator plumes) is also designed for the mostly-convective cases, because convective development is closely tied to the terrain. A comparison is made between measurements from an untreated period (NOSEED) and those from a treated period (SEED). The double differences of DOW reflectivity (SEED minus NOSEED in the target areas compared to the control areas) suggest a positive seeding impact in all the nine cases, but the distributions of precipitation enhancement of the mostly-stratiform cases and the mostly-convective cases are different. For the mostly-stratiform cases, more precipitation enhancement is found on the upwind side of the mountain crest, while for the mostly-convective cases, the seeding impact tends to be larger on the lee side. Also, the seeding mechanisms will be discussed, based on the DOW variables such as echo top, reflectivity, differential reflectivity and correlation efficient, the results suggest the main seeding mechanism tends to be "static seeding" (Bergeron process and deposition) for stratiform clouds. Moreover, reasonable correlations between the seeding impact and several related factors (liquid water path, untreated precipitation, stability, wind speed and temperatures) are also shown in this study, although the sample size is quite small.